foo.rb

class Foo
  attr_accessor :color
  attr_accessor :history

  def initialize
    @color = "white"
    @history = [@color]
  end

  def dump
    "color = <#{@color}>, history = <#{@history}>"
  end

  def reset_1
    color = "red"
    history << "red"
  end

  def reset_2
    self.color = "green"
    self.history << "green"
  end

  def reset_3
    @color = "blue"
    @history << "blue"
  end
end

foo = Foo.new
puts "start #{foo.dump}"
foo.reset_1
puts "reset_1 #{foo.dump}"
foo.reset_2
puts "reset_2 #{foo.dump}"
foo.reset_3
puts "reset_3 #{foo.dump}"

results in:

start color = <white>, history = <["white"]>
reset_1 color = <white>, history = <["white", "red"]>
reset_2 color = <green>, history = <["white", "red", "green"]>
reset_3 color = <blue>, history = <["white", "red", "green", "blue"]>